Hello, my name is Trent Elwing. I am an industrial hygienist at the Department of Labor & Industries – Division of Occupational Safety & Health. In April, 2011, I investigated an ammonia leak at a large warehouse where anhydrous ammonia is used in the refrigeration system in a cold storage building. In this incident, one worker was burned on the arm from the escaping gaseous ammonia. Anhydrous ammonia in refrigeration systems is pure ammonia in both a gas and liquid state and is much more dangerous than household ammonia dissolved in water.
As part of the planned mechanical integrity schedule, two employees were changing out the two pressure relief valves abbreviated PRVs shown inside the red circle that protected this ammonia compressor package. These PRVs prevent buildup of excessively high ammonia pressures which could burst piping, vessels or ammonia compressors. Ammonia inside the refrigeration system normally may have pressures as much as 10 times that of outside air. These PRVs are connected to piping painted orange which directs ammonia gas to a safe relief location on the building’s roof. During the leak, the corrosive ammonia peeled away the red floor paint.
This slide shows a close-up of the two PRVs that were being replaced. A 3-way valve separates the two PRVs. A 3-way valve allows isolation of one PRV for replacement while the second PRV continues to protect the refrigeration system from high ammonia pressures. By turning the 3-way valve stem completely clockwise, the PRV on the left is isolated and can be safely replaced. Having not received adequate training from their employer, the two employees attempted to, but ultimately did not isolate the left PRV. High pressure ammonia released violently into their breathing zones as they attempted to separate the PRV from the 3-way valve.
Before attempting the PRV changeout, both workers had put on full face air purifying respirators fitted with ammonia cartridges. When the ammonia gas suddenly released, both workers ran out the nearest exit door hitting the emergency stop button on their way out. A security guard was alerted and called 911. The respirators worn by the two workers provided both eye protection and limited protection from breathing ammonia gas. Neither worker reported breathing ammonia or ammonia burning their eyes. Although both workers were wearing polymer-coated mechanics gloves, one of the workers received a serious chemical burn from the concentrated ammonia contacting his arm above the glove’s cuff. He was taken to hospital for medical treatment.
An estimated 3600 pounds of ammonia evaporated into the air during this incident. Cold invisible ammonia gas escaped the compressor room and condensed water vapor creating a cloud that surrounded the compressor building. To protect themselves from chemical burns by the ammonia gas in the cloud, fire department emergency responders wore impermeable encapsulation suits. Inside these suits, the emergency responders breathed bottled air connected to their fullface respirator facepieces. For public safety, adjacent roadways were blocked off by the police. Before the fire department arrived, onsite employees had already stopped work and evacuated to a safe refuge away from the escaping ammonia.
Ammonia refrigerant is anhydrous ammonia and is strongly attracted to water and water vapor. Ammonia combines with skin moisture causing a stinging sensation and chemical burn. Inhaled airborne ammonia also burns wet surfaces of the respiratory tract and lungs. Excessive ammonia inhalation and/or skin contact can cause death. Airborne ammonia is considered to be immediately dangerous to life and health at 300 parts per million abbreviated ppm by volume of air. During the incident, airborne ammonia concentrations reached 1500 ppm outside the ammonia compressor building and 4000 ppm inside the compressor room. Fortunately the two workers escaped quickly before the ammonia cartridges of their air purifying respirators were overwhelmed.
All ammonia refrigeration systems have PRVs which need to be re-certified or changed out with a frequency of at least once every five-year period. Employers must ensure training, respiratory protection and personal protective equipment are provided to employees changing ammonia PRVs. Employers are also required to develop and implement an emergency response plan for uncontrolled releases of ammonia. Employers with larger systems containing more than 10,000 pounds of ammonia must comply with the Process Safety Management rule abbreviated PSM rule. Prior to the incident, this facility’s refrigeration system contained 27,000 pounds of ammonia. Deficiencies in the PSM program included inadequate training of the two workers on the PRV changeout procedure.
The responding firefighters, wearing self-contained breathing apparatus and encapsulation suits, entered the compressor room to stop the ammonia leak. Nearly 3 hours after it began and after two attempts, the leak was isolated. Employees changing out ammonia PRVs must take the precautionary steps of donning respiratory, eye and skin protection prior to PRV removal. Even with trained employees, failure of piping or other components can produce an uncontrolled ammonia release during PRV changeout. Let’s keep Washington Safe and Working by complying with the Process Safety Management rule and related rules for respirator and personal protective equipment use as well as emergency response to uncontrolled ammonia releases.